1. Field of the Invention
The present invention relates to a converter, called a broadcasting satellite (BS) converter in this field, which is used to receive BS signals in a satellite broadcasting system, and more particularly relates to an improvement of a control circuit incorporated in the BS converter to select either a high frequency band or a low frequency band included in a reception frequency band used in the satellite broadcasting system.
2. Description of the Related Art
A reception system of a satellite broadcasting system includes a low noise down converter block (LNB) provided in a parabola antenna, and a set top box (STB) connected to the LNB through the intermediary of a coaxial cable. In this specification, the LNB will be referred to as a broadcasting satellite (BS) converter, and the STB will be referred to as a broadcasting satellite (BS) tuner.
In recent years, a reception frequency band used in a satellite broadcasting system has been widened to accommodate digitization of the satellite broadcasting system and an increase in the number of channels thereof. For example, the widened reception frequency band is defined as one between 10.7 GHz and 12.75 GHz, and it is impossible to receive all broadcasting satellite (BS) signals (microwaves), included in the widened reception frequency band, with only one parabola antenna and one BS converter. In other words, it is necessary to prepare two parabola antennas and two BS converters before all the BS signals can be received and processed. Namely, the reception frequency band is divided into a low frequency band of 10.7 GHz to 11.7 GHz and a high frequency band of 11.7 GHz to 12.75 GHz, and the two parabola antennas and two BS converters are arranged for receiving and processing the respective low and high frequency bands.
JP-A-H08-293812, corresponding to U.S. Pat. No. 5,649,311, discloses a prior art BS converter which is constituted so as to receive and process all the BS signals included in the reception frequency band. Namely, according to JP-A-H08-293812, it is possible to receive and process all the BS signals with a single parabola antenna and BS converter.
This prior art BS converter is provided with a reception circuit for receiving and processing all the BS signals, and a control circuit for controlling the reception circuit. The reception circuit includes a mixer, and first and second local oscillators connected to the mixer. The first local oscillator inputs a first local frequency signal to the mixer, and the second local oscillator inputs a second local frequency signal to the mixer. The first local frequency signal features a lower frequency than that of the second local frequency signal. The control circuit selects which local oscillator should be driven.
In particular, when a television set, which is connected to the BS converter through the intermediary of the BS tuner and the coaxial cable, is tuned to a channel to receive a BS signal included in the low frequency band of 10.7 GHz to 11.7 GHz, only the first local oscillator is driven by the control circuit so that the BS signals included in the low frequency band of 10.7 GHz to 11.7 GHz are converted into intermediate frequency signals featuring a frequency of 950 MHz to 2150 MHz.
On the other hand, when the television set is tuned to a channel to receive a BS signal included in the high frequency band of 11.7 GHz to 12.75 GHz, only the second local oscillator is driven by the control circuit so that the BS signals included in the high frequency band of 11.7 GHz to 12.75 GHz are converted into intermediate frequency signals featuring a frequency of 950 MHz to 2150 MHz.
Thus, by using the prior art BS converter, it is possible to receive and process all the BS signals by the single parabola antenna and BS converter. Nevertheless, the prior art BS converter is not satisfactory in that it is impossible to obtain reliable operation.
In particular, when the television set is tuned to a channel to receive a BS signal included in the high frequency band of 11.7 GHz to 12.75 GHz, a band switching pulse signal featuring a duty factor of 50% is superimposed on a power supply voltage signal which is fed from the BS tuner to the BS converter through the coaxial cable. The control circuit includes a detector circuit for detecting whether the band switching pulse signal is superimposed on the power supply voltage signal, and a selector circuit for selectively driving the second local oscillator when the band switching pulse signal is detected by the detector circuit.
However, in this prior art, the detector circuit is susceptible to large amplitude noise, such as a spike noise or the like. As a result, a malfunction of the detector circuit may occur. Namely, the control circuit may mistakenly select which local oscillator should be driven, as explained in detail hereinafter.
Also, in the above-mentioned prior art, a band switching time, which is defined as a period of time measured from a time at which the television set is tuned to a channel to receive a BS signal included in the high frequency band to a time at which a picture is displayed on a screen of the television set based on the tuned channel, is relatively long. Namely, it takes a relatively long time to detect the band switching pulse signal by the detector circuit, as explained in detail hereinafter.